The present invention relates to a developing device for a copier, facsimile apparatus, printer or similar image forming apparatus and, more particularly, to a developing apparatus of the type having developer storing means storing a predetermined amount of developer, and a developer carrier to which the developer is supplied from the storing means. The developer carrier conveys the developer deposited thereon to an image carrier to develop a latent image formed on the image carrier.
An image forming apparatus of the type described is advantageously implemented with a developing device using a one component type developer, i.e., toner from the size, cost and reliability standpoint. Particularly, to effect color development, it is desirable to use a non-magnetic toner since this kind of toner is highly transparent. In a conventional developing device using a toner, a developer supply member is pressed against a developer carrier to supply the toner from developer storing means to the developer carrier. The developer carrier conveys the toner deposited thereon to an image carrier along a predetermined circulation path including a developing region. This type of device is taught in, for example, Japanese Patent Laid-Open Publication (Kokai) Nos. 60-229057 and 61-42672.
The prerequisite with the above-described type of developing device is that the image carrier and the developer carrier be rotated at substantially the same peripheral speed during the course of development. Otherwise, the toner would concentrate more in the trailing edge portion of an image than in the other portion. In the light of this, there has been proposed a developing device elaborated to deposit a required amount of toner and a required amount of charge on the developer carrier and the toner, respectively. For example, a developing device disclosed in Japanese Patent Laid-Open Publication No. 2-15110 has a developing roller, or developer carrier, whose surface is made up of a conductive portion connected to ground and dielectric portions distributed in the conductive portion in a regular or irregular pattern. The dielectric portions each has a small area. A sponge roller, or developer supply member, is rotatable in contact with the surface of the developing roller. A toner is frictionally charged by the two rollers contacting each other. At the same time, the dielectric portions of the developing roller are frictionally charged by the sponge roller and toner to form a great number of small closed electric fields, or microfields, in the vicinity of the surface of the developing roller. As a result, the frictionally charged toner is deposited on the developing roller in multiple layers by the microfields.
There has also been proposed a developing device having a toner container located at a higher level than a developing roller and partly or entirely implemented as a flexible member. Means for causing the flexible member to dent into the toner container is located to face the flexible member. With this arrangement, the device causes a toner aggregated on the bottom of the toner container to move positively. As a result, toner supply to the developing roller is prevented from failing due to the aggregation of the toner. This kind of scheme is taught in Japanese Patent Laid-Open Publication No. 4-152369 by way of example.
However, the developing devices disclosed in Japanese Patent Laid-Open Publication Nos. 60-229057, 61-42672 and 2-15110 have some problems left unsolved, as follows. To begin with, the sponge roller or similar developer supply member increases the cost of the device and needs an extra space for installation. Moreover, the toner supply member is driven in contact with the developing roller or similar developer carrier and, therefore, requires an extra driving force. Moreover, the force necessary for the developer carrier to be driven is increased due to the friction between the developer carrier and the developer supply member. Should the sponge roller be omitted to reduce the size and cost of the device, the toner would be aggregated below the developer carrier, resulting in short toner supply to the developer carrier. This is also true with the developing device proposed in Japanese Patent Laid-Open Publication No. 4-152369. In addition, the conventional devices frictionally charge the toner on the developer carrier, relying mainly on the developer supply member. Hence, when the developer supply member is omitted, the amount of charge to deposit on the toner becomes short.